Aside from undoing a downvote, if you just had a rep recalc, then deleted questions can cause small changes to your rep. There are three things that confer odd rep:

Question upvotes (+5, non-meta only)

Accepted answers (+15)

Downvotes issued (-1)

Undoing a downvote, or having a recalc done after the post you downvoted was deleted, will cause your reputation to change by exactly +1 if there are no other discrepancies.

You could also have a combination of the other factors. For example, if you asked a question that received 1 upvote (+5) and 3 downvotes (-6) and subsequently deleted it, you'd get +1 on the recalc.

It could also be that something was undeleted, which does happen. If you had an accepted answer on a question that was deleted (-15 on recalc), and two upvotes and downvotes (net score 0) on an answer that was undeleted (+16 on recalc), then that's also a change of +1.

I'm sure it sounds very improbable, but with enough users and posts it's bound to happen to someone.

When you hit the rep cap any upvotes will not have any effect on your reputation. However if you downvote a question and you loose -1 (200-1=199) from your reputation then if you get an upvote (within the same day), you will see a +1.